The invention relates to a matrix shim system for generating magnetic field components superimposed on a main static magnetic field parallel to a z-axis, wherein said magnetic field components act to homogenize the magnetic field component of the main static magnetic field along the z-axis in a volume of interest, wherein the volume of interest is centered at the origin of the z-axis, and wherein the matrix shim system comprises a plurality of annular coils, the axes of the annular coils coinciding-with the z-axis.
Such a matrix shim system is described in P. Konzbul et al., “Design of Matrix Shim Coils System for Nuclear Magnetic Resonance”, IEEE Transactions on Magnetics 36 (2000), No. 4, p. 1732-1736.
Nuclear magnetic resonance (=NMR) is a powerful tool in chemical analysis and imaging of samples. A sample is positioned in a strong static magnetic field and is subjected to electromagnetic pulses. The reaction of the nuclei of the sample is measured and analyzed.
The characteristics of the static magnetic field influence the quality of information that can be obtained from the sample. In general, best quality is achieved with high field strength and a high homogeneity (uniformity) of the static magnetic field.
The static magnetic field is generated by a main magnetic coil system, which is, in general, superconducting. The main magnetic coil system generates a main static magnetic field with a high field strength, but typically with an insufficiently low homogeneity. In order to homogenize the main static magnetic field, a shim system is used. The shim system generates magnetic field components which, when superimposed to the main static magnetic field, result in an overall static magnetic field with high homogeneity.
The main static magnetic field Bz in a z-direction can be described with a series expansion, with its summands being spherical harmonic functions Tnm multiplied with coefficients Anm, with n, m indices of summation. In typical cylindrically symmetric NMR geometries, m=0. The summand A00T00 describes the desired strong magnetic field, the summand A10T10 describes a field gradient that may be desired or not, according to the intended application and that may be dealt with a set of gradient coils, and all summands An0Tn0, with n≧2, describe undesired inhomogeneities.
Conventional shim systems consist of several coils arrangements, with each coil arrangement consisting of a coil or a group of coils. Each coil arrangement generates magnetic field components corresponding to one of the spherical harmonic functions. In other words, each coil arrangement may compensate for exactly one summand An0Tn0, with n≧2, assuming that an appropriate current is chosen within the coil arrangement, according to its so-called gradient strength. Each coil arrangement has its own current supply. The more coil arrangements used, the more orders of inhomogeneity which may be compensated.
However, these traditional shim systems are disadvantageous in that different coil arrangements (for different orders) would ideally occupy the same space within an NMR system. This may limit the achievable gradient strength of a coil arrangement, and it may limit, in the end, the number of orders of inhomogeneity that can be compensated for. Moreover, power efficiency in traditional shim systems is low since there are often windings of coils in close proximity to currents running in opposite direction, mostly canceling out each others' fields.
To overcome these disadvantages so called conventional matrix shim systems [5] are used. A matrix shim system has a plurality of coils, with each coil being fed by its own current source. Each coil contributes to several spherical harmonics. By carefully choosing the current (and thus the strength of the magnetic field generated) in each coil of the matrix shim system, it is possible to homogenize a main static magnetic field up to an order corresponding to the number of coils. When the number of coils exceeds the order of the main static magnetic field to be homogenized, additional conditions may be respected. Neighboring windings with oppositely running currents, as within a traditional shim systems, may be replaced with one single coil, for example. As a result, a matrix shim system is much simpler than a traditional shim system.
However, matrix shim systems also have disadvantages. Each coil has its strongest contribution to the resulting static magnetic field by an A00 summand. The A00 magnetic field component directly influences the measured spectra. This means that fluctuations in the power supply of any one coil cause side bands or increase the noise floor in measured NMR spectra.
Moreover, current sources, which are typically digitally controlled, may change their current value only in finite steps. Calculated current values for the coils must be rounded to feasible values. This causes significant deviations from the desired static magnetic field, in particular in the A00 and the A10 components.
It is accordingly the object of the invention to provide a shim system having the advantages of a matrix shim system, in particular simple coil geometries, the possibility to freely combine elementary fields, optimization of additional parameters, and at the same time providing very stable magnetic field components so a high quality of NMR measurements may be obtained.